1. Technical Field
This invention relates generally to composite structures and more specifically to a quasi-isotropic composite structure and method of making same.
2. Background Art
It is well known to make composite structures from woven materials, such as graphite or fiberglass threads or fibers, which are then bonded in a hard resin-like matrix to form a tough, high strength and lightweight member. An intrinsic property of these composite materials is that they expand and contract in different amounts depending on the orientation of the fibers since the coefficients of thermal expansion typically differ in the longitudinal and transverse directions. Consequently, when structures are made of these types of composite materials and then subjected to great variations in temperature, these traditional structures may tend to deform by warping and buckling. Such deformations might induce undesirable high internal stresses, especially if the structures are constrained. Furthermore, such part deformations are traditionally minimized by making the structure undesirably larger and heavier, thereby increasing raw material and manufacturing costs.
It is also known to employ composite isogrid structures made from layers of composite sheets which are formed into very thick graphite and epoxy flat stock. This flat stock is then milled into an isogrid configuration. However, this approach is costly since the high cost composite material is wasted in the milled out areas and since the machining process is very time-consuming and costly. Often, over 90% of the composite material is scrapped in this milling process.
Less efficient alternatives to an isogrid configuration have also been tried. For example, foam core or honeycomb structures have been used but may suffer from reduced heat transfer through the structure. These approaches also exhibit strength variations throughout the part. Other traditional constructions have attempted to adhere separate ribs onto composite isogrid structures, but with limited success. Structures having adhered ribs are known to have separation of the ribs from the face sheets.
Further examples of conventional composite structures are disclosed in the following U.S. Pat. Nos.: U.S. Pat. No. 5,554,430 entitled "Low CTE Boron/Carbon Fiber Laminate" which issued to Pollatta et al. on Sep. 10, 1996; U.S. Pat. No. 5,333,003 entitled "Laminated Composite Shell Structure having Improved Thermoplastic Properties and Method for its Fabrication" which issued to Archer on Jul. 26, 1994; U.S. Pat. No. 4,635,071 entitled "Electromagnetic Radiation Reflector Structure" which issued to Gounder et al. on Jan. 6, 1987; and U.S. Pat. No. 4,177,306 entitled "Laminated Sectional Girder of Fiber-Reinforced Materials" which issued to Schulz et al. on Dec. 4, 1979.
Autoclave molding is most commonly used during the curing of lay-up composites in order to provide high pressures to form dense parts; see, for example, U.S. Pat. No. 5,622,733 entitled "Tooling for the Fabrication of Composite Hollow Crown-Stiffened Skins and Panels" which issued to Asher on Apr. 22, 1997. A vacuum is often used to assist in the removal of trapped air or the like during the autoclave process. However, autoclave machines are very expensive to purchase and operate, especially for the manufacturer of low volume prototypes or production.
It is also known to provide other types of traditional composite isogrid members. For example, U.S. Pat. No. 4,086,378 entitled "Stiffened Composite Structural Member and Method of Fabrication" which issued to Kam et al. on Apr. 25, 1978, and U.S. Pat. No. 4,012,549 entitled "High Strength Composite Structure" which issued to Slysh on Mar. 15, 1977, disclose flanges or ribs disposed on composite structures.
3. Disclosure of Invention
In accordance with the present invention, an embodiment of a quasiisotropic composite structure includes a multi-sheet rib projecting generally perpendicular to a multi-sheet body with at least one of the body face sheets being bent and attached in a parallel manner to the rib sheet or sheets. A further aspect of the present invention provides that the sheet or sheets that are part of both the body and rib are unsevered and integrally contiguous. In another aspect of the present invention, each of the body sheets are made of composite material having generally unidirectional fibers. In yet another aspect of the present invention, each of the rib sheets are made of composite material having generally unidirectional fibers. The unidirectional fibers and sheets are oriented in an offset manner in relation to some of the other parallel stacked fibers and sheets so as to achieve the quasi-isotropic properties of the structure. Methods of making the composite structure of the present invention are also provided.
The quasi-isotropic composite structure of the present invention is highly advantageous over prior constructions since the present invention employs at least one common composite sheet for both an isogrid body and rib. This provides for superior strength, more uniform thermal expansion and superior securement of the rib to the body. This advantage is further heightened when a single contiguous and integral sheet is employed as part of two or more adjacent ribs and the body spanning therebetween. The present invention structure is also advantageous since it can be formed into a flat or curved isogrid structure. A curved isogrid structure additionally enhances the load bearing ability, especially as compared to traditional flat or even I-beam-type shapes, while allowing for a surface opposite the ribs to be free of ribs or other interruptions for receiving a mirrored coating or for providing a more aerodynamic shape.
The isogrid construction of the present invention is also more strength and weight efficient than would be traditional skin and stringer, milled or add-on rib configurations. The quasi-isotropic composite isogrid structure of the present invention is ideally suited for spacecraft, launch vehicles and aircraft where the significant weight savings of the present invention allow more payload to be delivered into orbit or improved fuel efficiency. The present invention structure and manufacturing method provide a generally non-deforming part that will retain its shape under a wide variety of temperature variations, including cryogenic temperatures, an the present invention apparatus is especially suited for use in constrained structures where the stresses induced by temperature variations will be exceptionally low as compared to conventional products and processes.
The method of manufacturing the present invention offers further advantages by allowing for low cost compression and curing of the composite structure in a simple oven rather than in a traditionally more expensive autoclave. Such a low cost manufacturing process is achieved by the use of blocks which have a differing thermal expansion rate as compared to a tool disposed on the opposite side of the composite structure, whereby the partially constrained blocks expand greater than the tool a nd compresses the adjacent portion s of the structure against the tool and/or each other when heated in the oven. Additional advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.